1. Field of the Invention
The present invention relates to improved Pauling Boilers for the concentration of sulphuric acid and to a process for the concentration of sulphuric acid in these improved Pauling Boilers.
2. Background Information
Waste sulphuric acid is frequently concentrated in grey cast iron boilers, so called Pauling Boilers, and thereby regenerated. This process is the object of DE-C 299 774. The waste sulphuric acid, which is at a concentration of about 70%, is introduced into a stripping column mounted on a vessel of grey cast iron which serves as still. The boiler, which is fired with gas or mineral oil, is filled with boiling, concentrated sulphuric acid which is drawn-off at the rate at which fresh acid is introduced and is cooled. The water present in the waste sulphuric acid leaves the stripping column as super-heated steam at 120.degree. C. to 175.degree. C.
Pauling Boilers are subject to corrosion on the inside due to the hot, concentrated sulphuric acid, but corrosion from outside in the firing chamber is particularly critical. The cast iron boiler develops scale according to the degree and distribution temperature in the firing chamber. Surface scaling is particularly severe when the output rate is high or when heat transfer is inefficient due to the deposition of iron sulphate on the internal wall of the boiler. The transfer of heat from the hot fuel gases to the boiling sulphuric acid becomes less efficient and requires progressively more energy input. The layer of scale also serves as an efficient insulating layer. If a scaled surface layer partially bursts off, thermal stresses are produced in the structure of the cast iron, which may cause cracking.
Further tensions and stresses in the periphery due to temperature changes as a result of starting and stopping of the operation and the diffusion of hot fuel gases into the structure are liable to deepen the cracks so that the cracks become visible on the surface. One disadvantage is that such structural changes and the cracks which inevitably form in the course of time can only be examined when the apparatus is cold. The risk of complete breakage of the boiler after a prolonged time in operation cannot be excluded.
Although scaling could be avoided by employing low surface temperatures, the production capacity would then be reduced and the process would become uneconomical.
It is therefore an object of the present invention to provide Pauling Boilers which do not have the disadvantages described above.